Page:Popular Science Monthly Volume 54.djvu/482

464 The sand grains of which the deposit is largely composed are seen to be arranged in layers nearly horizontal, and these layers are found to be due to alternations of sediment varying in fineness. This phenomenon is called stratification, and is what we should expect of the action of gravity operating on material of different sizes and densities suspended in a body of water. It has been found inexpedient to attempt to show a photograph of this section, owing to the smallness of the subject, but the same phenomena may be observed on a much larger scale in Fig. 5, which will be described below.

A few rods away the stream that feeds the pool has its origin. The sediment carried by the water and going to build up its delta has its source in part in a neighboring bank made up of material derived from solid rock by weathering, similar to that shown on our first excursion, and partly from older water deposits. Steep channels exist in the disintegrated rock, which represent the material removed by the fast-flowing rain water.

Now what geological phenomena have we observed at this locality? In the first place, it has become clear that running water possesses the power of transporting sediment. In the second place, this sediment has been deposited wherever the velocity of the water has been materially checked. The sediment has been laid down in horizontal layers under the influence of gravity. Furthermore, the material of which the delta is composed has been shown, in part at least, to have been derived from a solid rock such as forms our mountains. In our first excursion we saw that chemical change promoted disintegration; in our second, running water is observed seizing upon these products of decay, transporting them and building them into stratified deposits in the first convenient pool. A level-topped delta is first formed, which may or may not grow to fill the pool in which it is born. Some of the pools have become filled, while the delta as such has disappeared; it has grown into a tiny sand plain.

Let us see if the work performed by these temporary rivulets is typical of running water in general. For this purpose we shall visit a spot where a river enters some considerable body of water such as a lake. Let us inspect the river. Its water is sluggish, discolored by organic matter derived from decaying vegetation, and for some distance up stream from its mouth it meanders slowly across a flat, marshy area or meadow. If we also visit the spot at a time when the river is swollen by heavy rains or melting snows, the presence of this organic matter will be masked by the turbidity of the water; we shall learn that only in the freshet seasons does the water attain sufficient velocity to carry any visible load of sand and clay. The upper end of the lake will be found to be shallow, muddy, and water lilies will have discovered congenial surroundings. At another part of